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Old 08-21-2020, 10:16 AM   #1
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Correct Furnace size Townhouse in NH


I live in a two story Townhouse built in 1988 in southern New Hampshire.
Two walls are exposed to the outside, there are a total of 5 windows and 1 sliding glass door. The total size is 1250 sq feet.

I am replacing a 70K BTU unit, but the brands I am looking at don't make that size in 14" wide cabinets anymore.

I have talked to three contractors so far and they have all suggested different sizes. The first told me I would have no issue with a 50K unit. The second said a 60K unit would be enough, while the third says an 80K is the closest as the others are too small and while the 80K is a bit overkill it's not too much overkill.
The 80K would require a rebuild of the platform and some other accommodations.

Any thoughts? I am thinking the 50K is just too small myself.

Thank You,
Mark
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Old 08-21-2020, 10:18 AM   #2
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Re: Correct Furnace size Townhouse in NH


Have any of the contractors done a real load calculation?


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Old 08-21-2020, 10:31 AM   #3
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Re: Correct Furnace size Townhouse in NH


If the house is decent, a 40k at 96%+ efficiency (outputting 38k) should do the job no problem with plenty of capacity to spare. Doubt you have a heat loss greater than 30k btu/hr and it could easily be 25k.

Townhouses have very low heat loss.

Heck, many totally detached houses with R11 walls of that size in the northeast can and should be heated with that size furnace. Contractors are scared to downsize.

Do a load calc to tell to be sure -> https://www.loadcalc.net/, http://www.hvaccalc.com/

Why are you set on a 14" cabinet? A decent installer can use a transition to go from 17" to 14" no problem.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

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Old 08-21-2020, 10:46 AM   #4
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Re: Correct Furnace size Townhouse in NH


As jamal said, you need to have a real load calculation done. Chances are good the original unit was oversized (typical) but with a proper load calc you will be able to get a smaller unit with confidence.

Let me ask, what is wrong with the current unit?

I'm a retired energy auditor and can size heating systems but that is a job ALL hvac companies should be doing when selling a new system. The hvac calculation is called a "Manual J" and it can be done DIY especially with our help.

Bud
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Old 08-21-2020, 11:08 AM   #5
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Re: Correct Furnace size Townhouse in NH


Unless they’ve done a real manual J, all those contractors are guessing. If they can’t handle basic system sizing, what else will they do wrong?
www.loadcalc.net
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Old 08-21-2020, 12:11 PM   #6
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Re: Correct Furnace size Townhouse in NH


To answer some of the questions. The original unit is 16 years old and starting to fail. Already put over $1500 in repairs and don't want to go through another heating season.

As for why a 14" it's just that is what has always been there. It's a down flow and in a closet. There is probably enough room for the 17", but it is already a bit tight.

It's not so much the 17" vs 14" debate it's the I have to either go up or down as I can't get what I currently have BTU wise.

I would definitely like to see what the calc would be. Based on what I have found online already, but not with your help it comes out about 63000.

Looking over the link I am very confused.

BTW I am going 80% as I don't have the ability (cost or physical) to make the changes required to go 90. We are not allowed to make changes of any kind to the structure of the building.

Thanks,
Mark

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Old 08-21-2020, 12:24 PM   #7
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Re: Correct Furnace size Townhouse in NH


What calculations have you found online that indicate you need 63k BTU?
That link is a free version of a manual J.
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Old 08-21-2020, 12:25 PM   #8
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Re: Correct Furnace size Townhouse in NH


The extra cost of 90%+ is insignificant, it gets recouped over the life of the unit. A shame you have a condo corporation or hoa dictating what you can/can't do to your own property.

Size available depends on the brand. Most are made in 40-60-80, etc.

The btu output is what counts.

You may need to go up to 60k input (48k out) if 40k input (32k out) is not enough.

Some brands make them in other size increments.

To settle the sizing "debate", the calculations must be done properly.

If you have a/c, have to make sure the blower is sufficient. Some small units still come with 2 ton drives which work on paper but can really struggle to get air up to the second floor in the real world.


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I would definitely like to see what the calc would be. Based on what I have found online already, but not with your help it comes out about 63000.
BTU per sq ft as found on many bad websites is BS -> 63k for a small townhouse is nuts.
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Old 08-21-2020, 12:35 PM   #9
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Re: Correct Furnace size Townhouse in NH


I was looking over www.loadcalc.net and I am lost. I can get a few things, but most is beyond me. Is there a tutorial, especially for townhouses and how to deal with 2 walls that are not external? Also how to deal with two floors. I will give it a better look when I get home, but without help it's beyond me at this time.

Thanks,
Mark
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Old 08-21-2020, 12:42 PM   #10
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Re: Correct Furnace size Townhouse in NH


Shared walls and floors over heated areas are not included in the heat loss.

You have to know how the house was constructed - insulation r-values, wall thickness, window type, to do it properly.

Duct heat loss (say if the ducts are in an unheated vented crawlspace, insulation on them) must be taken into consideration.

May be worth paying a pro to do it.

Will probably end up being between 40 or 60k btu. If you have ducts in unconditioned space, too low velocity can increase heat loss.
Lots to consider.

The contractors are supposed to do this work but don't. in my area i don't think any residential contractor routinely does load calculations and evaluates air ducts.
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Old 08-21-2020, 01:14 PM   #11
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Re: Correct Furnace size Townhouse in NH


Quote:
Originally Posted by markrb View Post
I was looking over www.loadcalc.net and I am lost. I can get a few things, but most is beyond me. Is there a tutorial, especially for townhouses and how to deal with 2 walls that are not external? Also how to deal with two floors. I will give it a better look when I get home, but without help it's beyond me at this time.
Something messed up with that calculator. Just tried it myself. Says I need 127,000 BTU ( they mean perhaps BTU/hr ???? ).

I am losing 90,000 BTU (per hour, per week,... ???) through my floor which possibly would mean through my basement floor, which is 6 ft below grade (never freezes), and has 2" of sprayfoam under it.

Maybe I gotta find a better Manual J calculator. (Or buy a couple of wood stoves before winter comes.)
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Old 08-21-2020, 01:25 PM   #12
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Re: Correct Furnace size Townhouse in NH


What I would do is get a 60k 96% efficient 2 stage furnace. Most of the time it'll operate in first stage, easy and quiet.... but will have near the same capacity as your old unit on second stage for when the polar vortex pays a visit. You can keep fooling with the load calc stuff but a strategy like this should bracket you pretty good.
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Old 08-21-2020, 03:36 PM   #13
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Re: Correct Furnace size Townhouse in NH


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Originally Posted by raylo32 View Post
What I would do is get a 60k 96% efficient 2 stage furnace. Most of the time it'll operate in first stage, easy and quiet.... but will have near the same capacity as your old unit on second stage for when the polar vortex pays a visit. You can keep fooling with the load calc stuff but a strategy like this should bracket you pretty good.
Staging should not be used as crutch for sizing. Especially since this should be the installing contractors responsibly in the first place.
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Old 08-21-2020, 03:43 PM   #14
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Re: Correct Furnace size Townhouse in NH


OP said 90%+ isn't an option.

What u suggested outputs like 58k - way too much for a small townhouse, 2-stage feature would be wasted - end up with an expensive single stage.
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Old 08-21-2020, 03:45 PM   #15
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Re: Correct Furnace size Townhouse in NH


Quote:
Originally Posted by SPS-1 View Post
Something messed up with that calculator. Just tried it myself. Says I need 127,000 BTU ( they mean perhaps BTU/hr ???? ).

I am losing 90,000 BTU (per hour, per week,... ???) through my floor which possibly would mean through my basement floor, which is 6 ft below grade (never freezes), and has 2" of sprayfoam under it.

Maybe I gotta find a better Manual J calculator. (Or buy a couple of wood stoves before winter comes.)
Works okay it seems done properly.

It does say linear feet for slab which is different from sq ft.
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